1. Field of the Invention
The present invention relates to a mobile communication system, a mobile station, a base station and a communication control method where reservation based packet access for controlling radio parameters for a data packet by means of a reservation packet is employed in a multi-path fading environment.
2. Description of the Related Art
In the 3rd generation mobile communication systems, there is a significant increase in demand for data services using a wideband radio network. In order to satisfy this demand and to offer services with a lower cost and a faster information rate, high-speed access (broadband access) is increasingly needed.
In the case of uplink transmission, as for NRT (Non-Real Time) traffic such as downloading of large-volume data, demand for high-speed and high-capacity transmission can be considered to be lower than demand in the case of downlink transmission. However, as for RT (Real Time) traffic data with strict requirements for delay or as for interactive data services where uplink capacity and downlink capacity are symmetrical, it is necessary to increase link capacity for uplink transmission at the same level as that for downlink transmission.
Accordingly, as for data services where uplink capacity and downlink capacity are symmetrical, radio access is needed which achieves high quality reception in multi-path fading channels, low power consumption in a mobile station, flexible utilization of time/frequency/code resources, and so on.
In the aforementioned broadband packet radio access, on the other hand, it is expected that there will be a variety of requirements for QoS (Quality of Service) defined by a transmission speed, a transmission delay, an acceptable residual PER (Packet Error Rate), and so on.
As for RT traffic such as voice communications, video communications, and interactive services, for example, it is necessary to reduce an end-to-end transmission delay, because an increase in the transmission delay and delay jitter causes significant degradation in quality. In addition, as for NRT (Non-Real Time) traffic such as file transfer and WWW browsing, although requirements for the transmission delay are loose, high throughput and reliable transmission (error-free transmission in principle) is needed. Thus, technologies for controlling QoS are expected to be important in order to efficiently offer multimedia services with various requirements for QoS (for example, Non-Patent Reference 1).
In W-CDMA systems, random access based on slotted ALOHA is used for uplink transmission (for example, Non-Patent Reference 2). In this situation, either call requests and/or reservation control packets prior to a dedicated channel or discrete short packets are transmitted from a mobile station on RACH (Random Access Channel). In next-generation broadband packet radio access, it is expected that whole radio segments will be in the form of packet-based access.
It is also expected that there will be an increase in demand for burst Internet access. Thus, random (reservation based) access will be also expected to be more important, and efficient reservation based access in accordance with various QoS requirements is needed. Under this circumstance, reservation based access is proposed which controls a transmission power offset for a message data portion relative to a reservation packet portion in accordance with a QoS requirement (an acceptable delay) for traffic data to be transmitted by means of a reservation packet. Compared to the conventional method without packet retransmission-combining, the proposed reservation based access can reduce average received Eb/N0 (signal energy per bit-to-background noise power spectrum density ratio) which satisfies required quality, particularly for NRT traffic data where requirements for an acceptable delay are relatively loose, by reducing transmission power offset for each packet and utilizing a time diversity effect by means of packet retransmission-combining (for example, Non-Patent Reference 3).    [Non-Patent Reference 1] A. Harada, S. Abeta, and M. Sawahashi, “Adaptive radio parameter control considering QoS for forward link OFCDM wireless access”, IEICE Trans. Commun., vol. E86-B, no. 1, pp. 314-324, January 2003.    [Non-Patent Reference 2] 3GPP RAN, 3G TS 25.211 V3.4.0, September 2002.    [Non-Patent Reference 3] Y. Tizuka, M. Tanno, and M. Sawahashi, “Efficient random access channel transmission method using packet retransmission according to QoS”, IEICE Trans. Fundamentals, vol. E86-A, no. 7, pp. 1669-1675, July 2003.